Scandinavia is supposed to be a family-friendly paradise. We imagine fathers and mothers spending their children’s early months together at home. Then they enroll them in high-quality, government-subsidized child care, from which they pick them up at the end of the world’s shortest workdays.

But it is not as egalitarian as the fantasy suggests. Despite generous social policies, women who work full-time there are still paid 15 percent to 20 percent less than men, new research shows — a gender pay gap similar to that in the United States.

The main reason for this pay gap seems to be the same in both places: Children hurt mothers’ careers. This is, in large part, because women spend more time on child rearing than men do, whether by choice or not.

[Two line graphs show, "Children affect women's pay.. Earnings of Danish women, before and after having children", and, "But not men's pay... Earnings of Danish men, before and after having children", available to view online]

A series of recent studies shows that in both the United States and Europe, the gender pay gap is much smaller until the first child arrives. Then women’s earnings plummet and their career trajectories slow. Women who do not have children, by and large, continue to grow their earnings at a similar rate to men. There are still differences because of discrimination and other factors, but researchers say that motherhood explains a large amount of the gap.

It’s another sign that in modern economies, with their two-income families and with a priority on long hours spent in the office, even countries with the most family-friendly policies haven’t made things equal.

Policies like paid leave, subsidized child care and part-time work options are helpful to mothers. Scandinavia has one of the highest rates of women’s labor force participation in the world, and the share of women working in the United States has fallen behind the share in Europe, which has much more generous policies.

But policy alone would not be enough to overcome gender inequality. It would require changes in behavior — including by men. There is evidence that the gap would shrink if fathers acted more the way mothers do after having children, by spending more time on parenting and the related responsibilities.

“At the very least, men have to take a larger role,” said Francine Blau, an economist at Cornell who has studied the gender pay gap and family-friendly policies in the United States and Europe. “It does become a distinction in the eyes of employers between potential male and female workers, and it may reinforce traditional gender roles.”

One new study, which used a data set including everyone in Denmark from 1980 to 2013, along with details about their jobs and families, found that while there was a pay gap before people had children, it was relatively small and earnings were increasing at similar rates. But after the first child, women’s gross earnings quickly dropped 30 percent, and never fully recovered. In the long term, mothers earned 20 percent less. Women who did not have children continued to increase their earnings at a rate similar to men.

Most studies of the pay gap analyze equal pay for equal work. But in this paper, researchers examined how women changed their work in response to having children, and how that affected their lifelong pay. Mothers were paid less partly because they worked fewer hours, took longer breaks from employment and were more likely to move into lower-paying, family-friendly jobs, the paper found. Their probability of becoming a manager also declined.

“Equal work is in practice not an option for most women, because they have to take care of the children and therefore have different kinds of jobs and different kinds of hours,” said Henrik Kleven, an economist at Princeton, who wrote the paper with Jakob Egholt Sogaard, an economist at the University of Copenhagen, and Camille Landais, an economist at the London School of Economics.

As in the United States, the pay gap in Denmark has shrunk over time as women have become better educated than men and more likely to be professionals or managers. Children, which accounted for 40 percent of the pay gap in 1980, now account for 80 percent of it. Discrimination and other factors play a role in the remaining gap, researchers say.

The same pattern is true elsewhere. In the United States, a study by Census Bureau researchers found that between two years before the birth of a couple’s first child and a year after, the earnings gap between opposite-sex spouses doubles. The gap continues to grow for the next five years.

Two studies of college-educated women in the United States found that they made almost as much as men until ages 26 to 33, when many women have children. By age 45, they made 55 percent as much as men.

In Sweden, a recent study found, female executives are half as likely as men to be chief executives, and one-third less likely to be high earners — even when they were more qualified for these jobs than men. Most of the difference was explained by women who were working shorter hours and taking time off work in the five years after their first child was born.

As any parent knows, children come with a host of time-consuming responsibilities. Someone has to do the work. In most opposite-sex couples, that someone is the mother.

There are different explanations for this, researchers say. Women may have intrinsic preferences to do more of this work, or couples could decide it’s most efficient to divide the labor this way. It could also be that social norms about traditional gender roles influence men and women to behave this way.

In surveys of Americans and Europeans, people tend to say that women should work part-time or not at all when they have children at home, and that men should earn money to support their families. The Denmark study found evidence that women took on the roles they saw their mothers take — those whose mothers worked more had smaller pay gaps themselves.

Policies have different effects on how people approach work and home responsibilities, researchers say. Very long paid maternity leave, which is common in Europe, increases the chances that women return to the labor force but decreases their pay and promotions, because they take such long breaks.

Subsidized child care helps shrink the pay gap by enabling women to spend more time working. There is also evidence that mothers whose employers let them work flexibly or telecommute are less likely to reduce their work hours.

But as long as mothers, and not fathers, are the ones using policies like paid leave and taking on the additional work at home after having children, the lifetime pay inequity seems certain to remain.

Research has shown that when men take care of their babies in the early weeks, they are more involved in child care years later. But while Denmark gives new parents a year off, and mothers and fathers can share most of it, men on average take only two weeks, Mr. Kleven said. Countries like Sweden have been able to increase the amount of paternity leave that fathers take with a policy incentive: Families in which each parent takes a certain amount of time off receive additional time off to add to their combined allowance.

“If you know that both men and women will go off and take care of children, not just women, what that does is remove the motherhood penalty,” said Heejung Chung, a sociologist at the University of Kent.